What is a 3-way merge?

A 3-way merge combines two versions of a file using their common ancestor (the base) as a reference. By comparing each side against the base, the algorithm can tell whether a change came from A, from B, or from both — which a simple 2-way diff cannot. This is exactly how Git merges branches.

When does a conflict happen?

A conflict occurs when the same region of the base was changed differently by both A and B. If only one side touched a region, that side's version is taken automatically. If both made the identical change, there is no conflict — the shared result is used.

What do the conflict markers mean?

Conflicts are wrapped in Git-style markers: the lines after the opening marker are version A, the lines after the divider are version B, and the closing marker ends the block. Editing the output to keep one side and delete the markers resolves the conflict.

How are lines compared?

The tool splits each input into lines and runs a longest-common-subsequence diff of each side against the base, then walks the two diffs together to assemble stable regions, one-sided changes and conflicts. Whitespace and blank lines are treated as ordinary content.

Is my text uploaded anywhere?

No. The entire merge runs in your browser in plain JavaScript. No text is transmitted, so the tool works offline and your content stays private.

Text 3-Way Merge — Gera Tools

Combine two independently edited versions of a file using their common ancestor as a reference — the same 3-way merge Git performs when joining branches. Changes made on only one side are applied automatically; regions both sides changed differently are flagged as conflicts with Git-style markers. Everything runs locally in your browser.

How it works

A naive 2-way diff cannot tell whether a difference means “A added a line” or “B deleted a line”. A 3-way merge resolves this by using the base (common ancestor):

Diff base → A and base → B, line by line, using a longest-common-subsequence algorithm.
Walk both diffs in lockstep over the base lines, classifying each region:
- unchanged in both → keep the base text
- changed in A only → take A
- changed in B only → take B
- changed in both, identically → take the shared result (no conflict)
- changed in both, differently → emit a conflict

Conflicts are written with the familiar markers:

<<<<<<< A (mine)
A's version of the region
=======
B's version of the region
>>>>>>> B (theirs)

To resolve, edit the output to keep the side you want and delete the three marker lines.

Example

Base has the line port = 8080. Version A changes it to port = 9090; version B leaves it but adds debug = true below. These edits touch different lines, so the merge applies both with no conflict. If B had instead changed the same line to port = 7070, the region would conflict and be marked for you to resolve.

Tips

The cleaner your base, the fewer false conflicts: start from the exact version both edits diverged from.
Identical edits on both sides never conflict — the tool detects matching changes and keeps one copy.
After resolving, search the output for <<<<<<< to confirm no conflict markers remain before you copy.